I have a Zoomed 501 that I bought about three weeks ago and so far it's been nice. I don't hear it at all and seems to do a good job of filtering the water on my 6.6 gallon bookshelf aquarium. We'll see how it holds up over time, but initial thoughts are good.
One piece of advice, though, is to look in the box for the spray bar. The one I bought had two intake tubes, but no spray bar. When I went to exchange it for a new one, that one didn't have a spray bar, either. The guy at Petco did go get one from a 501 in the reptile department, so there seems to be an issue with the packaging of the 501 Nano, located in the aquarium filter section. The 501 in the reptile section is the same filter, just different packaging.